The invention relates to a method for measuring sorbate contents in substrates, especially of moisture contents in compost, and also a sensor for the practice of the method.
Sorbates, i.e., absorbed substances, especially water, constitute an important factor in the processing, transportation and storage of substrates. Thus, through the adjustment of the moisture in the substrate, a great influence can often be exercised on weight, volume and possible ongoing biological processes. Therefore a precise determination of the moisture in a substrate as well as in the environment, for example, is of great importance.
Unlike the measurement of sorbates in the environment, there are only a relatively small number of measuring methods available for determining the sorbate content of solids.
The method of the gravimetric determination of the moisture content of a substrate is, for example, the basis of the apparatus and corresponding method described in DE 38 19 335 A1. In this method samples are taken and then dried and volatile components are driven from the sample with heat, and the water is collected and its mass is determined. This method has the disadvantage that it is not possible to use it to obtain information for a desired prospective conduct of a process, which is essential for the economical operation of technically complicated composting installations. Another disadvantage of the gravimetric measurement process is that it measures not only the moisture content of the substrate, caused by physiochemical or mechanical absorption or binding (see e.g., A. W. Lykow: "Experimentelle und theoretische Grundlagen der Trocknung," VEB Verlag Technik Berlin, p. 62-63), which is biologically relevant, but it also includes in the measurement the chemically bound water, which is very tightly bonded and therefore not biologically relevant.
The known moisture sensors prove to be too inaccurate and unusable.
In the foods industry a tensiometric measuring process is known in which a sensor containing a measuring chamber is introduced into the substrate on a measuring lance, for example (W. Luck: "Feuchtigkeit - Grundlagen, Messen, Regeln" , R. Oldenbourg, Munich, Vienna, 1964, pp. 179-182). In the measurement of moisture, on the basis of diffusion of water molecules, an equilibrium between the water vapor partial pressure of the substrate and the water vapor partial pressure of the air as carrier medium takes place in the measuring chamber. After a certain time, therefore, the moisture of the air in-the measuring chamber is constant. At this moisture--the so-called equilibrium moisture--the air in the measuring chamber is in equilibrium, i.e., just as many water molecules are leaving the substrate as are entering the substrate.
This equilibrium moisture constitutes a specific value of the relative air humidity (ratio of absolute to maximum air humidity) from which the moisture content of the substrate can be deduced if the temperature of the substrate is known.
This process based on using the properties of the equilibrium moisture exists in different embodiments, as for example using also a surface sensor for flat substances. Such a system is described in DE 36 34 518 A1 among others.
The known tensiometric process has the advantage over the gravimetric process that the chemically bound water does not enter into the measurement. Instead, only the moisture content of the substrate is measured, which is biologically relevant. A disadvantage of the method of measuring moisture by means of the equilibrium moisture is, however, that it can be used only on hygroscopically moist substances, but not on wet substances, since the air above the latter is saturated with water vapor (see W. Luck, loc. cit.). Thus the known method has heretofore been used only in areas in which relatively low moisture contents are to be determined or controlled. This measuring method can be applied in the case of garbage compost only when the moisture content of the substrate is only up to about 40%. In the case of biocompost, measurements are possible only up to about 10% moisture content of the substrate. The reason for this is that above this substrate moisture content the air in the measuring chamber is saturated, as a rule, i.e., the relative air moisture amounts to 100%. Since the air is saturated, however, no conclusion as to the moisture content of the substrate is possible. In composting biogenic wastes, however, a compost moisture range between 50 and 60% is interesting, since the microorganisms work best under these ambient conditions.
On the other hand, however, it is desirable to perform the measurement of the moisture content of the substrate by measuring the equilibrium moisture so as not to include the chemically bound water in the measurement.